1. FIELD OF THE INVENTION
The present invention relates to a ball and socket joint particularly suitable for a prosthesis. More specifically, construction of the joint includes a relatively pliant socket material, while the ball has a hard, rigid surface. Unlike many prior art joints employing polymer liners, the relatively pliant material is selected from a material which is usually categorized as hard, such as metals. The benefit obtained by relative compliance is that when forces are imposed urging the ball against the socket, one contact surface yields and conforms to the other contact surface. Mutual contact area is thereby maximized, thus reducing maximal loading at any one localized point. When maximal localized loads are minimized, wear is mitigated.
A crucial combination of characteristics of the respective materials include specified values of surface hardness, tensile strength, smoothness, and breaking stress. Materials forming the contact surfaces are selected to further include biocompatibility, lack of cold flow, and susceptibility to advantageous forming techniques such as isostatic forming, rolling, and machining.
2. DESCRIPTION OF THE PRIOR ART
Prosthetic ball and socket joints are called upon to provide many qualities rendering them durable and suitable for their biological environments. Consequently, they are fabricated to precise dimensions from exotic, expensive materials. Also, they are implanted within a person in a demanding surgical operation, which also is expensive. Therefore, it is expensive and undesirable to require a replacement procedure in the lifetime of the patient. It follows that it is imperative that the joint last as long as is feasible.
Wear is a consideration in longevity of the ball and socket joint. Wear can modify the relative positions of the ball and socket, and can also release or generate debris in the form of particles, which can interfere with various body functions. Either of these two conditions may require that the joint be replaced with a new joint.
Another important consideration is that of progressive chemical deterioration of a component of the joint, particularly where mechanical removal of chemically formed films is a possibility. Chemical deterioration can lead to degradation of the smoothness of one surface or another. This has unfavorable implications for friction and consequent generation of localized heat and also for abrasion. Both localized heat and abrasion can reduce longevity of the implanted joint.
The wear aspect of joint operation has generally been addressed in the prior art by maximizing smoothness and hardness of the contact surfaces of the ball and socket members. An example is seen in U.S. Pat. No. 5,152,794, issued to James A. Davidson on Oct. 6, 1992. Davidson teaches coating of prosthetic surfaces with zirconium nitride and oxide. Unlike the device of Davidson, the present invention sets forth certain critical characteristics of materials selected for ball and socket components, and also specified quantitative values for these characteristics. U.S. Pat. No. 5,370,694, issued to James A. Davidson on Dec. 6, 1994, also discusses zirconium oxide and nitride, but sheds little if any further light on the novel parameters.
A method resulting in a titanium nitride surface of improved hardness and reduced friction is shown in U.S. Pat. No. 5,326,362, issued to H. Ravindranath Shetty et al. on Jul. 5, 1994. As in the case of Davidson, Shetty et al. is silent regarding the role of the combination of novel combination of characteristics, and upon the specified quantitative values thereof which form the thrust of the present invention.
A resilient member lining a rigid socket is shown in U.S. Pat. No. 5,425,779, issued to Marc H. Schlosser et al. on Jun. 20, 1995. However, unlike the present invention, the lining member is preferably made from ultra high molecular weight polyethylene (UHMWPE). Deficiencies of UHMWPE are noted in Davidson '794, and include, among other characteristics, creepage upon heating. By contrast, the resilient lining member of the present invention is made from a material not susceptible to creepage or flow.
U.S. Pat. No. 5,217,499, issued to Philp Shelley on Jun. 8, 1993, describes the socket of a ball and socket joint which has screw holes penetrating the socket member. However, Shelley fails to discuss the novel combination of characteristics, including the specified quantitive values thereof.
None of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed.